1. Field of the Invention
The present invention relates to a hot dip coating apparatus, as well as a method, for coating a steel sheet by using a coating bath of a molten metal. More particularly, the present invention is concerned with a hot dip coating apparatus and method in which a steel sheet is introduced into a bath of a molten metal through a slit formed in the bottom of a tank holding such a bath and pulled upward through the molten metal, while the bath of the molten metal is held without leaking through the slit by the effect of magnetic fields applied thereto.
2. Description of the Related Art
Hot-dip-coated steel sheets coated with various kinds of metals such as Zn, Al, Pb and Sn are finding diversified use, such as materials of automotive panels, architectural members, household electric appliances, cans, and so forth. A general description will be given of a process for producing a galvanized steel sheet, i.e., steel sheet coated with Zn, which is a typical example of the hot-dip-coated steel sheets. A cold rolled steel sheet is subjected to a pre-treatment in which the surfaces of the steel sheet are cleaned. The steel sheet is then heated and annealed in a non-oxodizing or reducing atmosphere, followed by cooling down to a temperature suitable for the hot dip coating, without allowing the steel sheet to be oxidized in the course of the cooling. The continuous steel sheet thus cooled is dipped in a bath of molten zinc. The steel sheet is then guided by rollers immersed in the molten zinc, e.g., sink rolls, so as to be pulled vertically upward out of the bath of the molten zinc. Any surplus molten zinc deposited on the surfaces of the steel sheet is removed by a doctoring device, such as a gas wiper, so that a suitable amount of the coating zinc remains on the surfaces of the steel sheet, which is then cooled.
This known method suffers from several problems caused by the presence of the immersed devices in the bath. First, the size of the tank containing the bath of molten zinc is inevitably large because of the presence of the immersed devices. The use of such immersed devices also restricts the selection and change of the type of coating molten metal. In addition, maintenance of the immersed devices is difficult. Furthermore, flaws or defects may appear in the surfaces of the product coated steel sheet due to introduction of dross into the nip of the sink rolls through which the steel sheet runs.
Accordingly, methods have been proposed for hot dip coating without the use of immersed devices, such as sink rolls. Among such proposed methods is "air pot method" that is capable of coating both sides of the steel sheet. As shown in FIG. 7, this method employs an apparatus which includes a coating tank for holding the molten metal bath and that has a slit in its bottom. A steel strip is introduced into the tank through the slit by being pulled vertically upward, so as to be coated with the metal of the bath. The coating apparatus further has an RF magnetic field application device 2b and a movable magnetic field application device, arranged as shown in FIG. 7, and further includes molten metal drain passage 11, molten metal supply passage 12, slit nozzle 20 and guide roller 33.
One of the critical requisites for the air pot method is a high degree of uniformity of the coating layer in the breadthwise direction of the strip. It is also important to ensure that there is no leakage of the molten metal through the clearance between the edges of the bottom slit and the surfaces of the strip running through the slit. Various measures have been proposed to meet these requirements by making use of an electromagnetic force. For instance, Japanese Patent Laid-Open No. 7-258811 proposes an apparatus in which a horizontal magnetic field is applied to the molten metal so as to hold the bath of the molten metal, while Japanese Patent Laid-Open No. 63-310949 discloses a method in which a bath of a molten metal is held by means of a linear motor. A method disclosed in Japanese Patent Laid-Open No. 5-86446 holds a bath of a molten metal by the combined effect of electromagnetic forces produced by an RF magnetic field and a movable magnetic field. In the method proposed in Japanese Patent Laid-Open No. 63-303045, molten metal constituting a bath is held by the effect of an interaction between a magnetic field and electric current and, at the same time, a gas jet seals the clearance at the slit through which the strip is introduced.
All these methods employ electromagnetic forces for the purpose of holding the molten metal without allowing the molten metal to leak through the clearances between the steel strip and the bottom slit through which the strip is steadily introduced and pulled upward. Such methods, however, have the following problems. The molten metal and the steel strip are induction-heated by electric currents induced therein as an effect of application of the electromagnetic fields, so that the temperatures of the molten metal and the steel strip are elevated undesirably. Such a temperature rise is notable particularly at the edges of the steel strip. The rise of the temperatures affects the reaction between the molten metal of the bath and the steel sheet in the bath, such that an alloy layer rapidly grows at the interface between the steel strip and the molten metal. The alloy is hard and fragile, so that an excessive growth of the alloy layer reduces the adhesion between the coating layer and the steel strip, permitting easy separation of the coating layer from the steel strip.
One commonly adopted technique to avoid this problem is to circulate the molten metal in the coating tank to prevent abnormal growth of the alloy layer caused by the rise of temperature of the molten metal or the steel strip. Such a circulation uses the molten metal as a cooling medium to prevent local build up of heat in the molten metal or the steel strip.
The molten metal is commonly circulated by continuously supplying the molten metal into the tank while discharging the same from the tank, as disclosed in Japanese Patent Laid-Open Nos. 5-86446 and 8-337875. However, continuous supply and discharge of the molten metal into and from the coating tank causes a variation of the flow velocity of the molten metal across the breadth of the steel strip, with the result that the dynamic pressure is locally elevated along the breadth of the steel strip. Leakage of the molten metal tends to take place where the dynamic pressure is high.
Circulation of the molten metal poses another problem in that separation of the coating layer is likely to occur due to the extraordinary growth of the alloy layer caused by lack of uniformity of the composition of the molten metal. The molten metal supplied into the coating tank inevitably contains components that suppress growth of the hard and fragile alloy layer at the interface between the coating molten metal and the steel strip. For instance, molten zinc used as the molten metal contains Al as the component for suppressing growth of the alloy layer. A variation of the flow velocity of the molten metal along the breadth of the steel sheet causes a corresponding variation in the effect of the alloy suppressing component along the breadth of the steel sheet, with the result that the growth of alloy layer cannot be suppressed satisfactorily where the flow velocity of the molten metal is comparatively low.
In most cases, the supply of molten metal into the coating tank is performed by a pump. Direct supply of the molten metal into the tank, however, creates a variation in the flow velocity of the molten metal in the breadthwise direction of the steel strip, particularly where the molten metal delivered by the pump is received. The above-described problems remain unresolved.
Japanese Patent Laid-Open No. 8-337858 discloses a hot dip coating technique in which molten metal is drained from a coating tank by overflow. This technique can provide a uniform distribution of flow velocity of the molten metal at the drained region where the molten metal is drained outside the coating tank, because the molten metal is allowed to overflow without encountering any obstacle. This technique therefore can effectively be used as a measure for suppressing local rapid growth of alloy layer, but is still unsatisfactory in that it cannot effectively suppress variation of the flow velocity of the molten metal where the molten metal is supplied into the coating tank. In other words, there is a demand for a technique that provides uniform flow velocity distribution of the molten metal in the breadthwise direction of the steel strip where the molten metal is supplied and where it is discharged.
The method in which a steel strip is introduced into a bath of molten metal through a bottom slit of a coating tank and pulled upward while the bath is held inside the tank by the action of electromagnetic force also faces the problem that, since the volume of the molten metal in the bath is extremely small, deposition of dross inside the tank becomes notable, particularly when the flow velocity of the molten metal varies along the breadth of the steel strip, tending to allow deposition of the dross on the steel strip.
The air pot coating method also suffers from the following problem. Vibration or other forms of spatial displacements may occur during steady coating operations causing the steel strip to fail to pass through the bottom slit of the tank cleanly, with resultant breakage of the edges of the slit or of the tank wall due to collision with the steel strip. Replacement or repair of damaged parts may be difficult and expensive.
One of solutions to this problem is to control the position of the coating tank in accordance with the position of path of the steel sheet so as to ensure that the steel strip always runs through the center of the slit formed in the bottom of the coating tank. This solution, however, is uneconomical because it is expensive. In addition, movement of the coating tank during the coating operation causes a vibration of the molten metal which renders the electromagnetic force temporarily ineffective, causing leakage of the molten metal through the slit. Leaking molten metal falls onto various components arranged along the pass line of the steel strip which is perpendicular to and right below the slit, such as deflector rollers of a steel sheet supporting device, support rollers for levelling the steel strip, guide rollers for suppressing vibration and so forth, so as to attach to these components. The coating metal attached to the path line components causes defects in the steel strip. Frequent cleaning, replacement or other maintenance work is required to prevent this problem.
Thus, some extraordinary conditions, such as extreme winding or vibration of the steel strip, hamper a stable and smooth coating operation. In order to deal with this problem, specific means for dealing with these extraordinary conditions are desired.
The methods that use electromagnetic forces to hold the bath of molten metal also suffer from a problem in that the molten metal tends to leak through the slit formed in the bottom of the coating tank during transitory periods, such as the period immediately after the start of supply of the molten metal into the coating tank or the period when the molten metal is drained after the coating operation is finished, because the effect of the electromagnetic force is insufficient to restrain the molten metal during the transitory period. Such leakage ceases when the electromagnetic force becomes large enough to hold the molten metal. However, the leakage of the molten metal through the slit before the electromagnetic force is large enough to hold the molten metal causes the same problems as described above in connection with the extraordinary conditions.